1. Field of the Invention
The present invention relates to method and apparatus for switching a yarn from a package to a fresh bobbin utilizing a so-called turret-type automatic yarn winder. Particularly, it relates to an improvement of the abovesaid method and apparatus regarding the acceleration of the package during the yarn switching operation.
2. Description of the Prior Art
Usually, a synthetic yarn spun from a spinneret is taken up on a bobbin as a package by a winder, in which the bobbin is held on bobbin holder and is driven by a surface contact with a rotating friction roller. When the package grows to a predetermined size, the running yarn is cut and temporarily sucked into an aspirator or a suction gun, during which the package is doffed from the bobbin holder and, in place, a fresh bobbin is donned thereto and, thereafter, the yarn is manually threaded from the aspirator to the empty bobbin to restart the take-up operation. However, such an operation consumes much suction air and produces considerable amount of waste material.
To eliminate the abovesaid drawbacks of the prior art, a so-called turret type automatic winder is proposed, for example, in Swiss Patent Specification No. 513,763 and U.S. Pat. No. 4,033,519. According to the turret type winder, a pair of bobbin holders are held on the opposite ends of a rotatable turret. Each bobbin holder is alternately displaceable from the normal winding position to the doffing position by every half a rotation of the turret. When the package is to be exchanged with an empty bobbin, the turret is made to rotate half a turn, whereby the package held on one bobbin holder is moved to the doffing position and, simultaneously, the empty bobbin held on the other bobbin holder is brought to the winding position. During the passage of this displacement, the yarn connected to the package is automatically transferred to the empty bobbin without using the aspirator or the suction gun.
The turret type winder is increasingly utilized for taking up a rather coarser yarn. However, when it is utilized for a yarn of medium or finer thickness, the yarn switching operation is not always successfully performed. That is, the yarn switching operation tends to fail when the size (weight) of the package to be doffed is largely deviated from the standard one.
The present inventors have found that the yarn tension during the yarn switching operation has a serious influence on the success rate of this operation.
As stated before, the turret-type winder is put to practical use for taking up the coarser yarn intended for industrial usage or for a tire cord. This is because such a yarn has a sufficient strength to be durable even against a larger fluctuation (particularly an increase) of the tension during the yarn switching operation and, therefore, a precise control of the tension is unnecessary.
A reason why the tension variation is generated is described below in detail. In this regard, though the explanation is made mainly on the friction drive winder in which the package is driven by surface contact with the friction roller positively driven at a constant peripheral speed, this principle is also applicable to other types such as a spindle drive type in which the bobbin holder itself directly drives the bobbin.
In general, a yarn tension in the normal winding operation of the friction drive winder is kept substantially constant. This is true in the case of the turret type automatic winder having a pair of bobbin holders. During the passage of rotation of the turret for exchanging the position of the package to that of the empty bobbin, the package on the one bobbin holder is detached from the friction roller at a certain angular position and, in turn, the empty bobbin on the other bobbin holder is brought into contact with the friction roller so as to start winding. In this stage, the yarn from the source such as a spinneret is still being wound on the package now being free from the friction roller. Therefore, if the package is kept free as it is, the winding speed of the yarn is gradually decelerated causing the lowering of the yarn tension. For enhancing the yarn switching operation, the yarn tension has to be kept in a preferable range. Thus, the acceleration of the package with a suitable timing is necessary or, otherwise the yarn switching operation tends to fail; that is, if the yarn tension is too high, the yarn is broken down before being transferred to the empty bobbin and, on the contrary, if too low, the yarn cannot engage with a yarn catching means provided on the empty bobbin such as a yarn catching groove or tape.
For smooth yarn switching in the conventional turret type winder, the empty bobbin is preliminarily accelerated by a suitable driving means to have a periphery speed of substantially the same as the yarn feeding speed and, thereafter, the package released from the friction roller is also accelerated by the same driving means now detached from the empty bobbin so that the yarn speed is controlled so as not to be greatly changed from the normal winding speed and the yarn tension is kept in a suitable range. Such the method is disclosed, for example, in Japanese Unexamined Patent Publication Nos. 49-100351 and 50-4342, and U.S. Pat. No. 4,033,519.
The abovesaid method may be effective when the timing for initiating the acceleration of the package is suitably selected. However, such a timing has to be modified in accordance with the size of the package. Therefore, if various size packages are sequentially treated in one winder, the success rate of the yarn switching operation tends to drop to a great extent. The recent synthetic fiber industry is required to manufacture many kinds of packages having various forms and dimensions in accordance with the market needs. Besides this, the package being wound on the winder often has to be exchanged for a fresh bobbin before it has reached the predetermined full size due to many reasons such as yarn breakage during the winding or waste winds in the start-up period, whereby the package subjected to the yarn switching may have various sizes in a range of from a bobbin with only a few yarn layers to a full package.
If the success rate of the yarn switching operation is low, there are required exclusive operators for the threading operation, which is contradictory to the object of the provision of the automatic winder, i.e., reduction of labour, and results in increased cost for the product.